The present invention is in the area of methods and apparatus for processing wafers as a step in manufacturing integrated circuits (ICs), and relates in particular to chemical vapor deposition (CVD) processes for depositing tungsten and alloys and mixtures of tungsten with other elements such as, in particular, silicon and nitrogen, using NF3 and in some cases, NH3 as a gaseous source of nitrogen in the processes.
Manufacturing of integrated circuits is historically a procedure of forming thin films and layers of various materials on wafers of base semiconductor material, and then selectively removing areas of the films to produce structures and circuitry. Doped silicon is a typical base wafer material, and in various process schemes, metal layers are formed on the doped silicon or on polysilicon or silicon oxide formed from the base material. It is well known in the art that there are many difficulties in forming thin metal films and in particular in forming such films on non-metallic base materials. Among these difficulties are problems of adhesion and problems related to diffusion and reaction of materials across material boundaries.
There are a number of well-developed technologies for deposition of materials in the ultra-thin layers required for IC fabrication schemes. The deposition techniques can be roughly classed as either physical vapor deposition (PVD) or Chemical Vapor Deposition (CVD) techniques. PVD processes include such processes as evaporation and re-condensation, wherein a material, typically a metal, is heated to a temperature at which the metal melts and vaporizes. The metal then condenses on surfaces generally in line-of sight of the evaporation, forming a film.
Another PVD process is the well-known sputtering process, wherein plasma of usually an inert gas is formed near a target material, and the target is biased to attract ions from the plasma to bombard the target. Atoms of the target material are dislodged by momentum transfer, and form an atomic flux of particles, which coalesce on surrounding surfaces generally in line-of-sight of the target surface eroded by the sputtering process.
PVD processes have distinct advantages for some processes, such as high rate of deposition, and relatively simple coating apparatus. There are drawbacks as well, notably an inherent inability to provide adequate step coverage. That is, on surfaces having concavities as a result of previous coating and etching steps, PVD processes are prone to shadowing effects resulting in local nonuniformity of coating thickness. This problem has grown in importance as device density has increased and device geometry has shrunk in size with multi-level interconnect schemes involving vias and trenches on microscopic scale.
CVD processes comprise deposition from gases injected into a chamber, wherein the gases or components of the gases are chemically decomposed and/or recombined by energy input. In typical CVD processes the substrate to be coated is heated, and gases introduced into a chamber holding the substrate react at or very near the substrate surface in a manner to deposit a film of material on the surface. For example, a film of metallic tungsten may be deposited on a heated substrate surface by flowing Tungsten hexafluoride to the surface in conjunction with a reducing gas such hydrogen. Resulting chemical reaction at a hot substrate surface reduces the tungsten hexafluoride, depositing a film of tungsten on the substrate, and producing HF gas.
It is well-known in the art that there are a wide variety of CVD processes known and available for semiconductor circuit processes, including deposition from organo-metallic materials and plasma-enhanced CVD, wherein energy is added to the process by exciting the gas above the deposition surface with a high-frequency discharge (plasma).
In the fabrication of an integrated circuit, transistors are developed on the surface of a doped silicon substrate. Once transistors are formed, to make a circuit, gates and drains have to be interconnected with electrically conductive tracks. This point in the overall IC fabrication process serves as perhaps the best example of a thin-film interface between a substrate material and an electrically conductive metal.
Over several years in the IC industry, a variety of materials have been tested for interconnecting tracks. Among these are aluminum, titanium. tungsten, and gold. Each has advantages and drawbacks, and different characteristics related to interaction with silicon, electrical conductivity, and electromigration, among others. Also, specific deposition processes have been developed suitable for specific materials. Aluminum, for example is typically deposited by the PVD process of sputtering, and there is currently no commercialized process for CVD deposition of aluminum as an interconnect material. A CVD process, on the other hand, can conveniently deposit tungsten.
There are a variety of known CVD processes employing variations of known chemistries to produce a variety of films of a single element, or a combination of two or more elements. Tungsten is deposited as a continuous (blanket) film on substrates for example to provide, after etching, both via plugs and interconnect tracks between devices implemented in doped silicon. Combinations of tungsten with other elements are deposited for other purposes, such as adhesion and barrier layers on gates of transistors implemented in silicon as an intermediate layer to improve adhesion and combat diffusion, for example, of silicon from the gates into the interconnect film. In these applications the films deposited directly on the gates prior to the interconnect material are called barrier layers.
One of the elements frequently combined with tungsten to provide specific desirable characteristics of a resulting film for adhesion and barrier purposes is Nitrogen to form tungsten nitride. The inventors are aware of conventional chemistry and processes for deposition of WNx (Tungsten Nitride). In some instances it is desirable to combine tungsten with other elements as well as Nitrogen. One such combination of interest and potential use in gate technology is Tungsten-Silicon-Nitride.
There are also elements competitive to tungsten for gate processes. One of these is Titanium, and materials of interest in combination with Titanium are Titanium Nitride (TiN) and Titanium Silicon Nitride (TixSiyNz).
The common element among the materials described above is Nitrogen, and perhaps the most common gas utilized as a source of Nitrogen in the CVD processes is ammonia (NH3). It is well known in the art that there are many problems with handling NH3 and mixing NH3 with other gaseous components for CVD processes. One such component is WF6 which, when combined with NH3 at room temperature produces an instantaneous and highly exothermic chemical reaction. Such a gas phase reaction is highly undesirable because it leads to serious complications in CVD reactor design and operation. Also an undesirable gas phase reaction leads to particulate formation, powdery deposits, and poor adhesion of films to the substrate. In addition to this, the instantaneous reaction between WF6 and NH3 leads to coating on the reactor (process chamber) walls. This coating contributes significantly to particles due to peeling, and hence reactors must be completely and periodically cleaned for operation in the actual production environment.
The processes with which the present patent application is concerned pertain to the art of depositing layers of materials typically less than 1 xcexcm thick, which are used to manufacture semiconductor integrated circuits. These processes are generally termed thin film deposition. The teachings of this disclosure address improvements to the art of chemical vapor deposition (CVD) for thin films consisting generally of the two elements tungsten and nitrogen.
The films in the processes of interest are typically deposited on silicon wafers placed in reaction chambers that provide a highly controlled gaseous ambient atmosphere. Inside these reaction chambers there is apparatus, such as a resistance heated plate, to heat the silicon wafers to provide energy to cause the gases to react to form thin layers on the wafer surface.
Process chambers for such processes are typically attached to a larger chamber, which contains a robotic transfer mechanism also operating under highly controlled ambient atmosphere, usually a high vacuum. The process chambers are typically arranged around a single centrally located robot to allow transfer of wafers between process chambers and load-lock chambers, which are isolated from the larger, general chamber by slit valves. Load-lock chambers are needed to allow introduction of the silicon wafers in to the controlled ambient. The arrangement often ends up looking like a cluster of smaller chambers surrounding a larger central chamber containing the robot, hence the popular term xe2x80x9ccluster toolxe2x80x9d to describe the type of equipment that is used often used
The generic terminology for thin films made up of tungsten and nitrogen is tungsten nitride. The actual films covered by the name tungsten nitride covers a wide range of nitrogen content and morphology. Nitrogen content can vary from a few atomic % to as much as 70 atomic %. The film morphology can range from a fully crystallized structure consisting of grains 10 nm and larger (polycrystalline) to an almost amorphous structure consisting of no long range order at all, or grains less than 1 nm, which is highly nano-crystalline.
Nitrogen in the tungsten nitride film can be bonded in a stochiometric arrangement with tungsten, and also concentrated at discontinuities in the film such as at grain boundaries. When crystallized at least the following phases can be present: W, W2N, WN, or WN2. The relative amounts of these phases in the film depends in part on the relative concentrations of W and N, which in turn can be controlled by adjusting the relative partial pressures of the source gases in the ambient atmosphere during deposition. The most stable phase of crystallized tungsten nitride is W2N. This is also the phase with the lowest electrical resistance which is an important parameter for many applications.
An important application of tungsten nitride films to semiconductor integrated circuit manufacturing is in the role popularly known as a barrier film. The use of barrier films occurs at many steps in the manufacture of semiconductor devices because of a need to achieve stable interaction between dissimilar materials on a microscopic scale, even when exposed to processing temperatures that can range at least as high as 1000xc2x0 C. The role of the barrier layer is to improve the following specific characteristics for a subsequent film deposition: promote nucleation, increase adhesion, and prevent intermixing. These attributes should in general be obtained by the barrier film while contributing minimally to increased resistance and thickness of the finished structure.
The transition metals in the periodic table are generally suitable as base material for barrier films based on their high melting points and suitable formation of a compound with silicon (silicide). To reduce intermixing due to diffusion aided by defects and grain boundaries the formation of a stable nitride is also a desired characteristic for a barrier film. Elements such as titanium, tantalum, and tungsten are good candidates.
Historically the most commonly used barrier film in the silicon-based semiconductor manufacturing industry is titanium nitride. Tungsten nitride was not thought to have the needed qualities needed by the industry. However, economic progress in the manufacturing industry of integrated circuits is based on continually shrinking the dimensions of the features on these integrated circuits. As the dimensions continue to decrease, the thickness allowed for a barrier layer also decreases. In addition new materials such as copper and tantalum pentoxide (Ta2O5) may be introduced in to the manufacturing process. These new materials require enhanced requirements of the barrier layers, such as very strong ability to block diffusion in the case of copper, and low tendency to remove oxygen in the case of tantalum pentoxide.
Based on the above, tungsten, as a denser material with lower tendency for oxidation than titanium, has become a potential candidate for several important applications in the industry. These applications include low resistance gate, capacitor electrode, and copper diffusion barriers.
Implementation of tungsten nitride in many potential applications is hindered by the common perception that suitable deposition methods for tungsten nitride may not exist. A survey of the literature of the deposition arts indicates very few papers depositing tungsten nitride by means other than physical sputtering. Few papers deal with chemical vapor deposition methods. Furthermore, the general conclusion of the art appears to be that chemical vapor deposition methods for tungsten nitride, especially those based on the simple reaction with ammonia are not suitable for manufacturing of integrated circuits, because of the extreme reactivity of the constituent gases in such processes that leads to particulate formation.
It is important to note that chemical vapor deposition methods as a class exhibit properties such as production stability and conformality in small features that are desired by the industry. The ability to deposit conformably in small features is very important, especially as the industry need is to shrink the dimensions of features on the integrated circuits. Therefore, it is significant that contrary to common teaching in the art, the present inventors have discovered principles and developed processes that provide several significant improvements to the methods of depositing tungsten nitride by chemical vapor deposition.
What is clearly needed are new processes for chemical vapor deposition to accomplish the needs of the industry in the several areas described, as the existing methods are not adequate.
In a preferred embodiment of the present invention a method for depositing Tungsten Nitride (WNx) using NF3 as a source of nitrogen is provided, comprising steps of (a) exposing a substrate surface to be coated in a CVD chamber, maintaining temperature of the substrate in a range of 300 to 450 degrees centigrade, inclusive; (b) flowing WF6 into the CVD chamber at a flow rate between 3 and 12 sccm inclusive; (c) flowing NF3 into the CVD chamber at a flow rate between 1.5 and 15 sccm inclusive; (d) flowing H2 into the CVD chamber at a flow rate between 200 and 500 sccm inclusive; (e) pumping the chamber to maintain a total pressure of from 50 to 500 mtorr; and (f) striking a plasma in the CVD chamber using a plasma power supply at a power level of from 50 to 300 watts inclusive. In different embodiments films of different thickness are produced by maintaining conditions listed for different lengths of time.
In some embodiments, in steps (b), (c) and (d) the elements are flowed to a premixing apparatus, and thence to the CVD chamber. In others there is a step for flowing argon to the CVD chamber to aid in supporting the plasma of step (f).
In another aspect of the invention a process for producing a substantially pure tungsten film is provided, comprising steps of (a) producing first a substantially amorphous tungsten nitride film on a substrate; (b) placing the substrate in a rapid thermal anneal (RTA) apparatus; and (c) establishing an ambient atmosphere other than substantially pure nitrogen or pure argon in the chamber, and annealing the tungsten nitride film by RTA at a temperature at or below 900 degrees centigrade. The other than substantially pure nitrogen or pure argon may comprise nitrogen with at least 5% hydrogen, or argon with at least 5% hydrogen, or H2O vapor with at least 5% hydrogen, or other mixtures of gases with hydrogen.
In yet another embodiment of the invention a process for controlling particle formation in a chemical vapor deposition (CVD) chamber using WF6 and NH3 gases as precursors is provided, comprising steps of (a) injecting the WF6 and NH3gases separately into the chamber; and (b) controlling the pressure at which the WF6 and NH3 gases meet and mix to less than 1000 mtorr.
In some embodiments there may be a further step for pre-coating walls of the CVD chamber with a hard-anodized aluminum film also substantially crack-free. In others a step for heating walls of the CVD chamber to a temperature between 70 and 90 degrees centigrade inclusive is used. In still other embodiments both steps for heating walls of the CVD chamber to a temperature between 70 and 90 degrees centigrade inclusive, and for pre-coating walls of the CVD chamber with a hard-anodized aluminum film also substantially crack-free may be used.
In still another embodiment of the present invention a process for controlling particle formation in a chemical vapor deposition (CVD) chamber using WF6 and NH3 gases as precursors is provided, comprising steps of (a) injecting the WF6 and NH3 gases into the CVD chamber; and (b) heating walls of the CVD chamber to a temperature between 70 and 90 degrees centigrade inclusive during CVD processing. There may be a further step in alternative embodiments for pre-coating walls of the CVD chamber with a hard-anodized aluminum film also substantially crack-free. In still other embodiments there may be another step for injecting the gases separately and controlling the pressure at the point of mixing to less than 1000 mtorr. In yet other embodiments there may be two further steps one for pre-coating walls of the CVD chamber with a hard-anodized aluminum film also substantially crack-free, and the other injecting the gases separately and controlling the pressure at the point of mixing to less than 1000 mtorr.
In still another embodiment a process for controlling particle formation in a chemical vapor deposition (CVD) chamber using WF6 and NH3 gases as precursors is provided, comprising steps of (a) injecting the WF6 and NH3 gases into the CVD chamber; and (b) pre-coating walls of the CVD chamber with a hard-anodized aluminum film also substantially crack-free. In alternative embodiments there may be another step for injecting the gases separately and controlling the pressure at the point of mixing to less than 1000 mtorr. In other alternative embodiments there may be a further step for heating walls of the CVD chamber to a temperature between 70 and 90 degrees centigrade inclusive during CVD processing. In still other embodiments both steps, one for injecting the gases separately and controlling the pressure at the point of mixing to less than 1000 mtorr, and the other for heating walls of the CVD chamber to a temperature between 70 and 90 degrees centigrade inclusive during CVD processing may be incorporated.
In still another embodiment a process for depositing Tungsten Nitride (WNx) using NH3 as a source of nitrogen is provided, comprising steps of (a) exposing a substrate surface to be coated in a CVD chamber, maintaining temperature of the substrate in a range of 300 to 450 degrees centigrade, inclusive; (b) flowing WF6 NH3, and H2 into the CVD chamber; and (c) striking a plasma in the CVD chamber. In this process in some embodiments the substrate temperature is maintained in a range of from 300 to 450 degrees centigrade, the WF6 is flowed to the chamber at from 3-12 sccm inclusive, the NH3 is flowed to the chamber in a range of from 1.5 to 24 sccm, the H2 is flowed to the chamber at from 200 to 500 sccm, and the chamber is pumped to maintain the total pressure below 1000 torr. In some embodiments the plasma may be pulsed.
In still another embodiment a process for forming a tungsten nitride film on an oxide surface of a substrate, with a graded interface of tungsten silicide at the oxide surface is provided, comprising steps of (a) heating a substrate having an oxide surface to a temperature of from 500 to 600 degrees Centigrade in a reactor chamber; (b) flowing WF6, NH3, H2 and SiH4 into a reactor chamber; and (c) pumping the chamber to maintain a total pressure of from 100 to 500 mtorr. In this embodiment WF6 may be flowed into the chamber at from 3-12 sccm inclusive, NH3 may be flowed into the chamber at from 1.5 to 50 sccm, and H2 may be flowed into the chamber at from 200 to 500 sccm.
The process improvements taught in enabling detail in the descriptions herein and below provide contributions in the art for improving processes in chemical vapor deposition for processes related t tungsten nitride, and afford new and better ways for manufacturers to produce integrated circuits.